In case information is read/written from/into a disk-like optical recording medium (referred to as an “optical disk” hereinafter) such as CD, DVD, or the like in the optical disk device, a track on the optical disk must be scanned by moving an optical pickup in a radial direction (traverse direction) of the optical disk while rotating the optical disk by a spindle motor. As a feeding mechanism for the optical pickup in the radial direction of the disk, a stepping motor is often used recently. The stepping motor is suitable for the feeding mechanism for the optical pickup because such motor can take a position at a fine rotation angle in response to an applied pulse train and also make a fast feed by feeding the optical pickup as the object of drive in terms of a direct drive.
In the stepping motor, a rotor is rotated at a rotation angle that is proportional to the number of pulses applied as a driving current. FIG. 15 is a view showing a revolving speed-torque characteristic of the stepping motor. In FIG. 15, an abscissa denotes a pulse rate of a drive pulse signal supplied as the driving current, i.e., a revolving speed of the stepping motor, and an ordinate a torque of the stepping motor in operation. In a range (self-starting range) Ti below a pull-in torque, the stepping motor can start, stop, and reverse in synchronism with an applied drive pulse signal. Also, in a range To in excess of a pull-out torque, an out-of-step phenomenon occurs, so that the stepping motor cannot take a rotating action in synchronism with the drive pulse signal. Also, in a range (through range) Ts between the pull-in torque and the pull-out torque, the stepping motor can continue to rotate in synchronism with the drive pulse signal when such motor has already been rotated, nevertheless an out-of-step phenomenon takes place and the stepping motor cannot make a normal rotation when such motor is started from its stationary state. Since the stepping motor has the above-mentioned characteristics, a design that is made based upon a load torque is absolutely essential to the stepping motor.
As the feeding operation of the optical pickup, there are an operation for performing intermittently a feed of about several tens μm in reading or writing the information from or into the optical disk (referred to as an “optical axis correction feeding operation” hereinafter) and a seek operation for performing a feed at a high speed toward the designated track over a long distance. In general, a full step drive (two-phase excitation drive) is executed in the seek operation that needs a fast movement of the optical pickup, while a microstep drive is executed in the optical axis correction feeding operation that needs a fine feed control of about several tens μm.
In the optical axis correction feeding operation, in order to scan the track on the optical disk, an objective lens of the optical pickup moves in the traverse direction following to the track that is recorded spirally or concentrically on the optical disk. At this time, a tracking actuator of the optical pickup executes position control of the objective lens with respect to the track of the optical disk. Since a movable range of the tracking actuator of the optical pickup is limited, the stepping motor is controlled in such a manner that, when the objective lens is shifted from a center of a base of the optical pickup by a predetermined amount, the base of the optical pickup is moved in about several tens μm in the traverse direction to cancel a shifted amount.
In the optical axis correction feeding operation, the stepping motor executes the feeding operation in the microstep drive. As shown in FIG. 16, in the microstep drive, a driving current like a sinusoidal wave is applied to respective terminals of the stepping motor to rotate the motor smoothly at a rotation angle smaller than a step angle peculiar to each motor. The microstep drive can make an infinitesimal feed of the optical pickup while suppressing generation of a vibration.
In the stepping motor, when the microstep drive is carried out in the form of continuous current supply, a consumption power is increased and also generation of heat is increased. Thus, such a problem arose that the life of parts is shortened or an operating temperature limit of the drive must be restricted. Therefore, the optical disk device for controlling the drive of the stepping motor by applying a driving current for the microstep drive only in predetermined time (several msec) is known (see Patent Literature 1, for example).
The optical axis correction feeding operation is continued during the reading or writing operation of the information from or into the optical disk, while the stepping motor is controlled such that the stepping motor is driven to cancel a shifted amount when a predetermined shift of the objective lens is generated. In other words, an operation of driving the stepping motor is repeated every time when a lens shift of the objective lens is generated, so that the information can be read correctly by driving intermittently the stepping motor.    Patent Literature 1: JP-A-10-149639